Mammalian chimeras and mosaics have been used for two types of embryological studies. The first, which is similar to classic experimental embryologic approaches, makes use of two cell populations to trace the origins and fates of cells by determining cell lineages in development. The second, which is a developmental genetics approach, analyzes how genetically different cells interact to form the tissues and organs of an adult. The genetic chimeras formed from preimplantation-staged mouse embryos have provided useful insights into mammalian embryogenesis particularly since the natural difficulties with viviparity preclude classical embryological approaches. From the studies performed with chimeric mice, a major limitation to their use in developmental studies is evident. This limitation results from the lack of a cell marker system that enables the unequivocal identification of cells of all genotypes or even of all cells of one genotype in histologic sections of chimeric tissues. Thus the location and distribution of chimerism within tissues cannot be topographically analyzed. If the full research potential of the chimeras is to be realized, such a system must be developed. The first aim of this proposal is the completion of an immunocytochemical adaptation that is designed to reveal the different cell populations in histologic sections utilizing antisera to genetically determined electrophoretic variants of an enzyme. The second aim of this proposal is to use this marker system to study cell lineage patterns of early embryonic stages, from the egg cylinder stage through the early somite stage, in chimeric mice. The general aim of this research program is to gain a greater understanding of the cell lineages, cellular interactions, and mechanisms involved in mammalian embryogenesis.